a) Field of the Invention
This invention relates to the structure of a spark plug, which is suitable for use in an internal combustion engine of an automotive vehicle or the like and has improved anti-fouling properties against the deposit of carbon on an insulator nose, especially at low temperatures.
b) Description of the Related Art
To prevent deposit of carbon on an insulator nose, especially at low temperatures when employed in an internal combustion engine of an automotive vehicle or the like, it has heretofore been the general practice to use a spark plug with a nichrome wire wound on and around an insulator nose which holds a center electrode in the vicinity of a free end of an axial bore. Because the nichrome wire is prone to oxidation and burning-up through its exposure to high-temperature combustion gas of a gas-fuel mixture, the spark plug is accompanied by the drawback that its service life is short. With a view to overcoming this drawback, spark plugs have been proposed, including a spark plug with a resistance heating pattern formed from a high m.p. (melting point) metallized ink on a surface of an insulator nose [Japanese Utility Model Laid-Open (Kokai) No. SHO 54-164322] as well as a spark plug formed by printing a resistance heating pattern layer with a high m.p. metallized ink of tungsten, molybdenum, platinum or the like on a surface of a green ceramic substrate of alumina or the like, forming an insulating covering layer by a ceramic sheet or paste of alumina or the like on the resistance heating pattern layer, winding the green ceramic substrate, which carries thereon the resistance heating pattern layer printed on its surface and covered by the insulating covering layer, on and around a nose of an insulator, and then simultaneously sintering the ceramic substrate and the resistance heating pattern layer together with the insulator to integrally bond the former to the nose of the latter [Japanese Utility Model Laid-Open (Kokai) No. SHO 55-10239].
These conventional spark plugs are however still accompanied by one or another drawback. In the case of Japanese Utility Model Laid-Open (Kokai) No. SHO 54-164322, the resistance heating pattern made from the high m.p. metallized ink is buried in the surface of the insulator nose. It is simple in structure but, as it is provided with almost no electrical insulation or only with incomplete electrical insulation for the protection of the heater itself, the electrical insulation easily fails during an operation of an internal combustion engine and a spark is hence produced between an associated center electrode and the resistance heating pattern formed from the high m.p. metallized ink and buried in the surface of the insulator nose. The spark plug therefore involves the drawback that the ignition of an air-fuel mixture may become insufficient.
In the case of Japanese Utility Model Laid-Open (Kokai) No. SHO 55-10239, on the other hand, with a view to fully ensuring protection and electrical insulation of the heater itself, the insulating covering layer is formed with the ceramic sheet or paste of alumina or the like on the resistance heating pattern layer after printing the resistance heating pattern layer on the ceramic substrate of alumina or the like with the high m.p. metallized ink of tungsten, molybdenum, platinum or the like. The insulating covering layer, which was made from the ceramic sheet or paste of alumina or the like and covers the resistance heating pattern printed on the ceramic substrate, is however susceptible to breakage due to increased combustion gas pressure produced in an associated combustion chamber as a result of the recent move toward high-performance internal combustion engines. The insulating covering layer can no longer maintain sufficient electrical insulation, leading likewise to the drawback that no full ignitability can be retained for an air-fuel mixture.